This invention relates to an X-ray fluorescent image intensifier and, more particularly, to improvements in an input section of such intensifier.
A usual object observation system using an X-ray fluorescent image intensifier is as shown in FIG. 1. As is shown, ahead of X-ray tube 1 is disposed X-ray fluorescent image intensifier 2. X-rays having been transmitted and modulated through object 3 are incident on X-ray fluorescent image intensifier 2. An output image of X-ray fluorescent image intensifier 2 is picked up by a television camera (not shown) to be reproduced on a monitoring television (not shown).
X-ray fluorescent image intensifier 2 has input screen 4 provided at the front end and output screen 5 provided at the rear end and facing input section 4. In the operation of X-ray fluorescent image intensifier 2, the modulated X-ray image on input screen 4 is converted into optical image and then into a photoelectron image. The photoelectron image is focused and accelerated to reach output screen 5, at which an optical output image with intensified brightness can be obtained. This optical output image is picked up by a television camera, for instance.
The input screen of such a prior art X-ray fluorescent image intensifier 2 has a structure as shown in FIG. 2. As is shown, on the concave surface of aluminum substrate 6 having a spherical surface is formed phosphor layer 8 consisting of columnar crystals 7 of sodium iodide-activated cesium iodide phosphor. Intermediate layer 9 consisting of an aluminum oxide layer and an indium oxide layer is formed on phosphor layer 8, and photocathode 10 is formed on phosphor layer 9.
In an object observation system using the above X-ray fluorescent image intensifier, it is desired to reduce the amount of X-rays illuminating object 3. In order to obtain satisfactory brightness and resolution with such a small quantity of X-rays, it is necessary to permit X-rays having been transmitted through object 3 to be incident on the phosphor layer without loss to increase the absorbed X-rays. To this end, the quantity of X-rays absorbed in aluminum substrate 6 is as small as possible, and it is most desirable to omit aluminum substrate 6. With the prior art screen structure, however, it is impossible to omit aluminum substrate 6.
In order to increase the quantity of X-rays absorbed in the phosphor layer, columnar crystals 7 desirably have as large length as possible. Where the length of columnar crystals 7 is increased, however, the number of times of refraction of light in phosphor layer 8 is increased to increase the quantity of light propagated from the side surface of a columnar crystals to an adjacent one. This reduces the resolution. For this reason, the length of columnar crystals 7 can not be increased too much, and its upper limit is approximately 400 .mu.m.
Further, with the prior art phosphor layer 8 phosphor is evaporatedly deposited on the concave surface of aluminum substrate 6, so that the grown columnar crystals 7 are directed in directions crossing the central axis of aluminum substrate 6. Since this direction crosses the direction of incidence of X-rays, with increase of the length of columnar crystals 7, in peripheral portions of the input screen a plurality of columnar crystals 7 adjacent to one another are caused to fluoresce simultaneously with incidental X-rays on the same route. Thus, the resolution is reduced. Further, since intermediate layer 9 is an evaporated layer consisting of aluminum oxide and indium oxide, it has a large number of light reflection points to reduce the resolution.
Further, phosphor layer 8 consisting of columnar crystals 7 has inferior light transmittance compared to the phosphor layer formed by the melting, so that the sensitivity is inferior. Further, the phosphor layer 8 consisting of columnar crystals 7 has a large number of fine surface irregularities, so that electrons from photocathode 10 formed on phosphor layer 8 are emitted in various directions. Therefore, the electrons are not satisfactorily focused, and the resolution is reduced.
Further, scattered X-rays radiated from object 3 and evacuated envelopes in the neighborhood of input screen 4 are absorbed in columnar crystals 7 of phosphor layer 8 to reduce the contrast.
To solve the above problems, there has been proposed a fluorescent image intensifier having an input phosphor screen, which consists of a honeycomb-like supporting plate of a heavy metal having a plurality of apertures defined by partition walls and phosphor material filling the apertures (as disclosed in Japanese Patent Disclosure No. 55-21805). According to this publication, the honeycomb-like supporting plate is formed with holes using an electron beam or a laser beam. With this method, however, a processing time of 2,600 hours or more is required for manufacturing a honeycomb-like supporting plate with a diameter of 12 inches, for instance. This is impractical.